This announcement offered a rare glimpse of a long-running campaign to sabotage Iran's critical nuclear installations, believed to be a key priority of Western and Israeli intelligence agencies.

The Fordow plant, located inside a bunker dug into a mountainside, became the latest target on Aug 17 when an explosion severed its electricity cables, running from the nearby city of Qom.

Fereydoun Abbasi, the head of the Iranian Atomic Energy Organisation, disclosed the incident during a speech in Vienna to the 155 members of the International Atomic Energy Agency (IAEA). He noted that IAEA inspectors visited Fordow the day after the explosion and insinuated that they might have been responsible.

"During the early hours of next morning, an Agency inspector requested to conduct an unannounced inspection. Does this visit have any connection to that detonation?" asked Mr Abbasi. "Who, other than the IAEA inspectors, can have access to the complex in such a short term to report and record failures?"

Mr Abbasi added: "Terrorists and saboteurs might have intruded the Agency and might be making decisions covertly."

Five Iranian nuclear scientists are known to have been assassinated since 2007. Mr Abbasi was himself wounded when a motorcyclist attached a bomb to his car in Tehran in November 2010, on the same day as another scientist was killed by this method.

The Fordow enrichment plant, buried beneath about 260ft of rock and earth, was built in secret from 2006 onwards. But Western intelligence discovered its construction, allowing President Barack Obama to reveal the installation's existence in 2009. Fordow is Iran's most valuable plant because its location in a hollowed out mountainside could render it immune to air attack.

The IAEA says that 696 centrifuges are being used to enrich uranium inside Fordow, with another 1,444 installed but not yet operational. If these machines lost their power supply, they would be severely damaged, said Mark Fitzpatrick, the head of non-proliferation at the International Institute for Strategic Studies.

"If there was no power source to keep them spinning, and if they stopped, as they slowed down they would crash," he said.

But IAEA inspectors visited Fordow the day after the incident, on Aug 18. Their report on Iran's nuclear programme, released on Aug 30, does not mention any damage to the centrifuges.

This suggests that Fordow must have a backup electricity system. It would, in any case, be an extraordinary oversight for an installation of this sensitivity to lack an independent power supply and rely on a normal grid connection.

The development of Fordow may not be progressing as rapidly as Iran might have hoped. While the total number of centrifuges installed in the plant has tripled since February, the number of operational machines has remained constant for the last seven months at 696.

Mr Fitzpatrick noted that the electricity supply for Iran's other enrichment plant at Natanz had been singled out for sabotage, with one power surge destroying 50 centrifuges. It was "entirely conceivable" that Fordow could be encountering the same attention, he said.

The CIA is understood to have begun a sabotage campaign, code-named "Olympic Games", under the Bush administration. The most successful intervention was the Stuxnet computer virus, which makes centrifuges spin out of control and tear themselves to pieces. This was infiltrated into Natanz in 2009 and briefly forced all enrichment to be halted for emergency repairs. This virus alone probably delayed Iran's nuclear ambitions by up to a year.